1. Field of the Invention
The present invention relates to an electroluminescent display device, and in particular, to transistors constructing the circuit structure in the pixel section of an electroluminescent display device.
2. Description of the Related Art
An electroluminescence (hereinafter referred to as EL) display device which uses an EL element which is a self-illuminating element as an illumination element in each pixel has attracted a strong interest as an alternative display device for a display device such as a liquid crystal display device (LCD) and a CRT because the EL display device has advantages such as thin width and low power consumption, in addition to the advantage of being self-illuminating. Such an EL display device has thus been researched.
In particular, there is a high expectation for an active matrix type EL display device in which a switching element such as, for example, a thin film transistor for individually controlling an EL element is provided for each pixel and EL elements are controlled for each pixel, as a high resolution display device.
FIG. 1 shows a circuit structure for one pixel in an active matrix type EL display device having m rows and n columns. In the EL display device, a plurality of gate lines GL extend on a substrate in the row direction and a plurality of data lines DL and power supply lines VL extend on the substrate in the column direction. Each pixel has an organic EL element 50, a switching TFT (first TFT) 10, an EL element driving TFT (second TFT) 20, and a storage capacitor Cs.
The first TFT 10 is connected to the gate line GL and data line DL, and is turned on by receiving a gate signal (selection signal) on its gate electrode. A data signal which is being supplied on the data line DL at this point is then held in the storage capacitor Cs connected between the first TFT 10 and the second TFT 20. A voltage corresponding to the data signal is supplied to the gate electrode of the second TFT 20 via the first TFT 10. The second TFT 20 then supplies a current, corresponding to the voltage value, from the power supply line VL to the organic EL element 50. In this manner, the organic EL element in each pixel is illuminated at a brightness based on the data signal, and a desired image is displayed.
The organic EL element is a current-driven element which is illuminated by supplying a current to an organic emissive layer provided between a cathode and an anode. The data signal output onto the data line DL, on the other hand, is a voltage signal with an amplitude corresponding to the display data. Thus, conventionally, in order to accurately illuminate the organic EL element by such a data signal, in an organic EL display device, a first TFT 10 and a second TFT 20 are provided in each pixel.
The display quality and reliability of the organic EL display devices described above remain insufficient, and the characteristic variations in the first and second TFTs 10 and 20 must be dissolved. In particular, reduction in characteristic variation in the second TFT 20 for controlling the amount of current supplied from the power supply line VL to the organic EL element 50 is desired, because such variation directly causes variation in the illumination brightness.
Moreover, it is preferable to construct the first and second TFTs 10 and 20 from a polycrystalline silicon TFT which has quick operation speed and which can be driven by a low voltage. In order to obtain a polycrystalline silicon, an amorphous silicon is polycrystallized by laser annealing. Because of various reasons such as, for example, energy variation in the irradiating laser at the irradiation surface, the grain size of the polycrystalline silicon is not uniform. When grain size is not uniform, in particular around the TFT channel, there is a problem in that the on-current characteristic or the like of the TFT may also vary.
The present invention is conceived to solve the above problem, and one object of the present invention is to improve reliability in a device having an organic EL element or the like as an element to be driven.
According to one aspect of the present invention, there is provided a semiconductor device comprising a thin film transistor for controlling the power supplied to an element to be driven, provided between the element to be driven which operates based on the supplied power and a power supply line for supplying power to the element to be driven, wherein the thin film transistor and corresponding element to be driven are electrically connected to each other by a wiring layer; and the contact position between the wiring layer and the thin film transistor is placed to be distant from the contact position between the wiring layer and the element to be driven.
By placing the contact position between the wiring layer and the thin film transistor distant from the contact position between the wiring layer and the element to be driven, the formation of the element to be driven, which in many cases is formed at a layer above the wiring layer, on a more flat surface is facilitated. The thin film transistor and the wiring layer are separated by an insulation layer and the contact between the thin film transistor and the wiring layer is achieved through a contact hole formed on the insulation layer. The connection between the wiring layer and the element to be driven is achieved through a contact hole formed on another insulation layer for insulating the wiring layer and the element to be driven. Therefore, when the contact hole for connecting the thin film transistor and the wiring layer is formed to overlap to the position of the contact hole for connecting the wiring layer and the element to be driven, the element to be driven, which is formed at the uppermost layer, would be formed on top of an uneven surface with a large height difference due to two (steps of) contact holes. When an emissive element, for example, an organic EL element which uses an organic compound in the emissive layer, is used as the element to be driven, electric field concentration or the like tends to occur at the layer which includes the organic compound if the flatness of the formation surface is not good, and a dark spot which cannot illuminate tends to be generated from such electric field concentration. Therefore, by placing at a distance the contact between the wiring layer and the element to be driven from the contact between the thin film transistor and the wiring layer, the flatness of the formation region of the element to be driven can be improved.
According to another aspect of the present invention, in the semiconductor device, it is preferable that the element to be driven is an emissive element which includes an emissive element layer between a first electrode and a second electrode; a contact hole is formed on an insulation layer which is formed above the wiring layer; the wiring layer is connected through the contact hole to the first electrode of the emissive element which is formed on top of the insulation layer and covering the contact hole; at least the contact hole region of the first electrode is covered by a flattening layer for planarization; and the emissive element layer is formed above the first electrode and the flattening layer.
By covering the contact hole region of the first electrode by a flattening layer, that is, by filling the recessed section caused by the presence of the contract hole by the flattening layer, a surface which has a very high flatness can be obtained by the first electrode and the flattening layer. Thus, by forming the emissive element layer on the surface with very high flatness, the reliability of the element can be improved.
According to another aspect of the present invention, there is provided a semiconductor device comprising a thin film transistor for controlling power supplied to an element to be driven which operates based on the supplied power and which includes an emissive element layer between a first electrode and a second electrode, the thin film transistor being provided between the element to be driven and a power supply line for supplying power to the element to be driven, wherein the thin film transistor and the corresponding element to be driven are directly or indirectly, and electrically connected to each other at a contact hole formed on an insulation layer for separating the thin film transistor which is formed at a lower layer and the element to be driven; at least the contact hole region of the first electrode is covered by a flattening layer; and the emissive element layer is formed above the first electrode and the flattening layer.
An emissive element layer is formed above the first electrode, but, because the recess section on the first electrode generated by the presence of the contact hole is covered by a flattening layer, the first electrode and the flattening layer can create a very flat surface even when the recess section is deep such that the reliability of the element can be improved by forming the emissive element layer on top of the surface with a high flatness.
According to another aspect of the present invention, it is preferable that the element to be driven is an organic electroluminescence element which employs an organic compound as an emissive layer. Although such an organic EL element has high brightness and wider selection ranges for the illumination color and material, because the organic EL element is current driven, variation in the amount of supplied current causes a variation in the illumination brightness. By using the circuit structure of the pixel or placement as described above, it is possible to easily maintain uniformity of the supplied current. In addition, by employing the placement and structure of the contact points as described above, the aperture ratio can be increased and the element layer such as the emissive layer can be formed on a flat surface, and a more reliable element can be obtained.